1. Field of the Invention
The present invention relates generally to an exhaust gas purification system for an internal combustion engine and particularly to an apparatus for controlling introduction or feeding of fresh air into an exhaust system of an internal combustion engine for promoting purification of engine exhaust gas through catalytic reaction. In particular, the present invention is concerned with an improved method and apparatus for controlling introduction or feeding of fresh air into an exhaust gas purification system of an internal combustion engine for purifying effectively the exhaust gas by eliminating hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NO.sub.x) through catalytic chemical reaction even in the warming-up state of the engine. Further, the invention is concerned with a method and apparatus for controlling optimally the flow of air fed into an exhaust gas purification system in dependence on engine operation states which are effective not only for suppressing discharge of nitrogen oxides (NO.sub.x) as well as carbon monoxide (CO) and hydrocarbon (HC) to the atmosphere but also for enhancing fuel cost performance of the engine while protecting the catalytic converter against degradation due to overheating. Additionally, the invention is concerned with an apparatus for detecting occurrence of abnormality in a fresh or secondary air feeding control system adapted for controlling air flow introduced into an exhaust system of an internal combustion engine while heating the air.
2. Description of the Related Art
The regulations imposed on the exhaust gas control for the automobiles becomes more and more severe. Under the circumstances, lot of efforts have heretofore been paid to development of techniques for removing noxious gas components contained in the exhaust gas discharged from the internal combustion engines. In this conjunction, it is known to employ a reduction catalytic converter for removing nitrogen oxides (NO.sub.x) and an oxidation catalytic converter for eliminating carbon monoxide (CO) and hydrocarbon (HC) through catalytic chemical reactions. It is further known to introduce or feed fresh air into an exhaust pipe of an internal combustion engine (hereinafter also referred to simply as the engine) for the purpose of promoting purification of the engine exhaust gas through the catalytic chemical reactions even in the state immediately after the start of engine operation where the temperature and purification efficiency of the catalytic converter is still low. For a better understanding of the present invention, the background techniques and the related art will first be described in some detail.
FIG. 27 shows in a schematic diagram a structure of an internal combustion engine system equipped with an apparatus for controlling an amount of air to be introduced or fed into an exhaust pipe of the engine. Referring to the figure, the engine 1 from which noxious gas components are discharged as the result of combustion of a fuel mixture is provided with an intake pipe 2 having an air cleaner 3 installed at an inlet port for filtering out dust and foreign particles from the air as it is taken in. An air flow sensor 4 is disposed at a location downstream of the air cleaner 3 in the intake pipe 2 for detecting flow rate of the air supplied to the engine 1. Further, installed within the intake pipe 2 is a throttle valve 5 for regulating the amount of air to be charged into the engine 1 by varying the cross-sectional area of the intake pipe 2. For supplying fuel to the engine 1, fuel injectors 6 are installed in an intake manifold of the engine 1 for atomizing and ejecting fuel fed through a fuel pump (not shown) toward associated intake valves (not shown) of the engine 1, respectively. An exhaust pipe 7 connected to the engine 1 for discharging the exhaust gas therefrom to the atmosphere is equipped with a catalytic converter 8 for purifying the exhaust gas through chemical reaction. An air feeding system for introducing or feeding fresh air into the exhaust pipe 7 by bypassing the engine 1 is arranged between the intake pipe 2 and the exhaust pipe 7 and includes an air feed pipe 10 having one end connected to the intake pipe 2 at a position downstream of the air cleaner 3 and the other end connected to the exhaust pipe 7 at a location upstream of the catalytic converter 8 for forcively feeding the air into the exhaust pipe 7 through an air pump 11 installed in the air feeding pipe 10. In order to prevent the exhaust gas from flowing reversely into the intake pipe 2 by way of the air feeding pipe 10, a check valve 12 is mounted in the pipe 10. Further, the air feeding pipe 10 is equipped with an electric heater 13 for heating the air to be fed into the exhaust pipe 7. An air-fuel ratio sensor 14 which may be constituted by a zirconia oxygen sensor including a zirconia element or a titania oxygen sensor composed of a titania element is disposed at a position corresponding to a function between the exhaust pipe 7 and the exhaust manifold for detecting concentration of oxygen contained in the exhaust gas. A fuel controller 15 is connected to the air-fuel ratio sensor 14 and the fuel injectors 6 for controlling the latter in dependence on the output of the air-fuel ratio sensor 14. Additionally, connected to the heater 13 and the fuel controller 15 is a heater controller 16 for controlling the heater 13.
Next, description will turn to operation of the system described above. A fraction of the air taken in and purified through the air cleaner 3 is forcively diverted into the air feeding pipe 10 through the air pump 11 of the air feeding system 9 to be heated by the heater 13 and then introduced through the check valve 12 into the exhaust pipe 7 at a location upstream of the catalytic converter 8. The air fed to the exhaust pipe 7 in this way is mixed with the exhaust gas of the engine 1. The resultant gas mixture is then subjected to chemical reactions within the catalytic converter 8, whereby the noxious exhaust gas components HC (hydrocarbon), CO (carbon monoxide) and NO.sub.x (nitrogen oxides) are converted into H.sub.2 O and CO.sub.2. The engine exhaust gas thus purified is then discharged to the atmosphere.
The fuel controller 15 determines an amount of the fuel to be injected in the engine 1 on the basis of the engine speed (rpm) and other operation parameter. Besides, the fuel controller 15 corrects the fuel injection amount thus determined in accordance with data derived from the output signal S1 of the air-fuel ratio sensor 14 to thereby generate a signal S2 for driving the fuel injector 6 such that the air-fuel ratio assumes a predetermined value such as a stoichiometric ratio. More specifically, the fuel controller 15 serves to control the fuel injection in accordance with the oxygen (O.sub.2) content of the exhaust gas through a feedback loop.
Introduction of the fresh air into the exhaust pipe 7 through the air pump 11 is started substantially at the same time as the start of the engine operation and continued until the engine is stopped. In other words, the air pump 11 operates continuously during operation of the engine 1 to feed incessantly the fresh air into the exhaust pipe 7.
As is apparent from the above, with the arrangement of the exhaust gas purification air feeding control system for the internal combustion engine known heretofore, concentration or content of oxygen in the exhaust gas is detected by the air-fuel ratio sensor 14, wherein the air-fuel ratio of the air-fuel mixture which is to undergo combustion within the engine is so controlled by the fuel controller 15 as to assume a predetermined value, e.g. a value close to a stoichiometric or theoretical air-fuel ratio which can ensure a maximum purification efficiency for the catalytic converter 8. However, the exhaust-gas purification air feeding control system suffers a problem. Namely, operation of the air pump 11 is continued even after the air-fuel ratio control by the fuel controller 15 has been strutted, whereby the air is continuously fed into the exhaust pipe 7 with the flow rate being maintained substantially constant. As a consequence, when the air-fuel ratio is controlled through the feedback loop to the stoichiometrical value at which the catalytic converter 8 can exhibit the optimal purification characteristic, as mentioned previously, there takes place an oxygen-in-excess state in the area upstream of the catalytic converter 8, as a result of which oxygen molecules in excess oxidize those fractions of carbon monoxide (CO) and hydrocarbon (HC) molecules which are to partake in the reaction for reducing NO.sub.x (oxides of thus nitrogen), incurring shortage of reactants for the reduction of NO.sub.x. Ultimately, the NO.sub.x -purification efficiency of the catalytic converter 8 becomes lowered, whereby the amount of NO.sub.x discharged to the atmosphere increases undesirably.
To cope with the problem mentioned above, it is certainly conceivable to control the amount of air introduced into the exhaust pipe 7 as a function of the air-fuel ratio determined by the fuel controller 15. In that case, however, since the amount of air as fed into the exhaust pipe 7 can be controlled only after the air-fuel ratio control has put into effect, it is impossible to detect the ratio between the engine exhaust gas and the fresh air in the gas mixture introduced into the catalytic converter before the air-fuel ratio control is started. In other words, it can not be determined whether or not the optimal reactions take place in the catalytic converter 8 nonetheless admixture of the fresh air with the engine exhaust gas.
Another problem of the exhaust gas purification air feed system known heretofore can be seen in that because the system is provided for the purpose of activating the catalytic converter 8 as early as possible for enhancing the exhaust gas purification efficiency, occurrence of failure or malfunction in the exhaust gas purification air feed system will naturally result in discharge of undesirable gas components in the atmosphere. In order to solve this problem, it is necessary to detect occurrence of abnormality as early as possible and generate an alarm to the driver. However, the exhaust gas purification air feed system for internal combustion engine known heretofore is not in the position to solve this problem.
Heretofore, it is further known to dispose a first catalytic converter for reduction of NO.sub.x and a second catalytic converter for oxidation of HC and CO in the exhaust pipe of the engine in combination with the provision of the fresh air feeding system (also known as the secondary air flow system), as is disclosed in Japanese Unexamined Utility Model Publications No. 21018/1972 (JP-UA-47-21018) and Japanese Unexamined Patent Application Publication No. 60019/1984 (JP-UA-59-60019).
More specifically, a catalytic converter for reduction and a catalytic converter for oxidation are serially installed in the exhaust gas pipe of the engine in this order as viewed in the direction of the exhaust gas flow, wherein in the engine operation state which follows immediately the start of the engine and in which hydrocarbon and carbon monoxide occupy a large proportion of the exhaust gas, the fresh air (also referred to as the secondary air) is introduced to the reduction-oriented catalytic converter which is thus placed in an oxidizing atmosphere to thereby remove hydrocarbon (HC) and carbon monoxide (CO).
For more particulars of the fresh air feeding apparatus known heretofore will be described below by reference to FIG. 28.
As can be seen in FIG. 28, the engine system is comprised of an internal combustion engine 201, a transmission 202 operatively connected to an output shaft (not shown) of the engine 201, an intake pipe 203 for charging air into the engine 201, an exhaust pipe 204 for discharging engine exhaust gas, a three-way (reduction) catalytic converter 205 and an oxidation catalytic converter 206 disposed in the exhaust pipe 204, respectively, for purifying the exhaust gas, a mechanical air pump 209 for taking in and supplying fresh air to the exhaust gas purification system, an a fresh air feeding pipe 210 for introducing the air taken in through the air pump 209 to the catalytic converters 205 and 206, respectively. A throttle valve 207 is disposed in the intake pipe 203 with an air cleaner 208 being mounted at an inlet port thereof.
In operation of the engine 201, the exhaust gas flows through the exhaust pipe 204 to be purified through the three-way (reduction) catalytic converter 205 and the oxidation catalytic converter 206. At that time, the fresh air is introduced to both the reduction catalytic converter 205 and the oxidation catalytic converter 206 by way of the air feeding pipe 210, as a result of the which excess oxygen state prevails within the exhaust pipe 204 as well as the converters 205 and 206, whereby hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gas become more susceptible to oxidation and thus concentrations of these noxious components are decreased.
The conventional apparatus described above also suffers a problem that although removal of HC and CO components is promoted because the catalytic converter 205 is exposed to the oxidizing atmosphere, nitrogen oxides (NO.sub.x) are difficult to be reduced through the converter 205, resulting in that the quantity or concentration of NO.sub.x discharged to the atmosphere can not be decreased. Further, since the fresh air (also referred to as the secondary air) is at a lower temperature than the exhaust gas in the starting operation state of the engine, activation of the catalysts is accompanied with a time lag, leading to degradation in the purification efficiency of the catalysts.
In general, when the air-fuel ratio of the gas mixture charged in the engine is so controlled as to be substantially constant in the vicinity of the theoretical or stoichiometric air-fuel ratio (typically 14.7), the efficiency of the exhaust gas purification system including one or more catalytic converters is maintained optimal for oxidation and reduction of noxious components such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x), whereby these noxious gas components can be removed substantially completely before being discharged to the atmosphere. A problem of such exhaust gas purification system is also seen in that when the air-fuel ratio of the mixture charged into the engine is so controlled that the fuel content is high as experienced in the engine starting phase and in the high load state of the engine, content of oxygen in the air-fuel mixture charged in the engine becomes insufficient for oxidation of HC and CO, resulting in that the engine exhaust gas discharged from the engine contains significant amounts of HC and CO components. To cope with this problem, there has heretofore been proposed an exhaust gas purification system for allowing the noxious gas components such as CO and HC to be eliminated by feeding fresh air into the exhaust pipe, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 65541/1984 (JP-A-59-65541).
This known exhaust gas purification system will be reviewed below by reference to FIG. 29 which shows in a block diagram a structure of an internal combustion engine system equipped with an apparatus for controlling an amount of air to be introduced or fed into an exhaust pipe. Referring to the figure, an internal combustion engine 301 is comprised of, for example, four cylinders and provided with an intake pipe 305 having an air cleaner 302 installed at an inlet port for filtering out dust and foreign particles from the intake air. As a result of combustion within the engine, noxious gases such as HC and CO are produced when oxygen is insufficient, while NO.sub.x (nitrogen oxides) is produced when oxygen are in excess and engine is operating at a high temperature.
An air flow Sensor 303 is disposed at a location downstream of the air cleaner 302 in the intake pipe 305 for measuring flow rate of the air supplied to the engine 301. Further, installed within the intake pipe 305 is a throttle valve 304 for regulating the amount of air to be charged into the engine 301 by varying the cross-sectional area of the intake pipe 305. For supplying fuel to the engine 301, fuel injectors 310 are installed in an intake manifold of the engine 301 for atomizing and ejecting fuel fed through a fuel pump (not shown) toward associated intake valves (not shown) of the engine 301. An exhaust pipe 306 connected to the engine 301 for discharging the exhaust gas therefrom is equipped with a catalytic converter 307 for promoting purification of the exhaust gas by removing the noxious gases through chemical reaction.
An air feeding system (also referred to as the secondary air feeding system) for introducing fresh or secondary air into the exhaust pipe 306 is arranged between the intake pipe 305 and the exhaust pipe 306 and includes an air feed pipe 305A having one end connected to the intake pipe 305 at a position downstream of the air cleaner 302 and the other end connected to the exhaust pipe 306 at a location upstream of the catalytic converter 307 for forcively feeding the fresh air into the exhaust pipe 306 through an air pump 308 installed in the air feeding pipe 305A. In order to prevent the exhaust gas from flowing reversely into the intake pipe 305, a check valve 309 is mounted in the air feeding pipe 305A.
The rotation speed of the air pump 308 is controlled by a control signal C2, whereby the flow rate of the fresh or secondary air as introduced is controlled correspondingly.
An air-fuel ratio sensor 311 is mounted in the exhaust pipe 306 for detecting concentration of oxygen contained in the exhaust gas, to thereby output a detection signal AF, the voltage level of which changes with reference to a level indicating an oxygen concentration substantially corresponding to the stoichiometric air-fuel ratio (14.7).
A fuel controller 312 is connected to the fuel injector 310 for controlling the fuel injector 310 and the air pump 308 in dependence on the output of the air-fuel ratio sensor 311 and engine operation state signals including signals indicative of the intake air flow rate, temperature of the engine cooling water, engine speed, throttle opening indicative of engine load and others which are detected by associated sensors, respectively.
The fuel controller 312 for generating the control signal C1 for the injectors 310 and the control signal C2 for the air pump 308 is implemented in the form of a microcomputer imparted with arithmetic function and serves for arithmetically determining a target fuel amount in dependence on the output signal AF of the air-fuel ratio sensor 311 and the engine operation state signals generally designated by D to thereby generate the control pulse signal C1 having a duty cycle corresponding to the target fuel amount, whereby the air-fuel ratio of air-fuel mixture charged into the engine is so controlled as to be substantially equal to the theoretical or stoichiometric air-fuel ratio.
FIG. 30 is a waveform diagram for illustrating a relation between the fuel amount charged into the engine and the output signal AF of the air-fuel ratio sensor 311.
As can be seen from the figure, the air-fuel ratio detection output signal AF of the air-fuel ratio sensor 311 rises up steeply when the air-fuel mixture fed to the engine is rich (i.e., when the air-fuel ratio is smaller than the stoichiometric ratio of 14.7) while the former falls steeply when the latter is lean (i.e., when the air-fuel ratio is greater than the stoichiometric value of 14.7). Thus, the fuel amount injected through the injector 310 is so controlled as to be decreased when the signal AF indicates that the air-fuel mixture as charged is rich and vice versa.
Next, description will turn to operation of the engine system described above by reference to FIGS. 29 and 30. A part of the air taken in through the air cleaner 302 is forcively diverted into the air feeding pipe 305A through the air pump 308 to be introduced through the check valve 309 into the exhaust pipe 306 at a location upstream the catalytic converter 307.
The air fed to the exhaust pipe 306 in this way is mixed with the exhaust gas of the engine 301. The resultant gas mixture is then subjected to oxidizing reactions in the catalytic converter 307, whereby the noxious exhaust gas components HC (hydrocarbon) and CO (carbon monoxide) are oxidized to H.sub.2 O and CO.sub.2, respectively. The engine exhaust gas thus purified is then discharged to the atmosphere.
At this juncture, it is to be noted that the air-fuel ratio sensor 311 is usually constituted by an O.sub.2 -sensor and thus detects the concentration or content of O.sub.2 in the mixture gas resulting from the mixing of the engine exhaust gas and the fresh or secondary air supplied through the air feeding pipe 305A. The air-fuel ratio detection signal outputted from the air-fuel sensor 311 is supplied to the fuel controller 312 together with the engine operation state signal D.
The fuel controller 312 fetches the air-fuel ratio detection signal AF and the engine operation state signal D to thereby determine the basic amount of fuel to be injected through the injector 310 in dependence on the engine operation state indicated by, for example, the intake air flow rate and/or the engine speed and corrects the basic fuel amount as determined by taking into consideration the air-fuel ratio detection signal AF, whereby the corresponding control signal C1 is generated for controlling the fuel injector 310. Further, when the engine is in the state of low or intermediate load, the control signal C2 is generated for operating the air pump 308. In this case, the control signal C1 indicates the air-fuel ratio of the air-fuel mixture to be charged into the engine which is slightly smaller than the stoichiometric ratio (i.e., the air-fuel mixture is rich more or less). However, the output AF of the air-fuel ratio sensor 311 indicates that the air-fuel mixture as charged into the engine is lean because the oxygen concentration of the exhaust gas is high due to addition of the fresh air supplied from the air pump 308.
When it is decided by the fuel controller 312 on the basis of the eigne operation state signal D that the engine 301 is in a high load state, the controller 312 interrupts the control signal C2 to thereby stop the operation of the air pump 308. The feeding of the fresh or secondary air into the exhaust pipe 306 is thus interrupted, whereby the concentration of oxygen contained in the exhaust gas diminishes because of no introduction of the fresh air into the exhaust system. As a consequence, the fuel controller 312 controls the fuel injection such that the air-fuel ratio of the mixture to be charged into the engine becomes slightly greater than the stoichiometric ratio (i.e., the mixture becomes lean).
As is apparent from the above, when the fresh air is fed to the exhaust system in the engine state of low and/or middle load in order to promote purification efficiency of the catalytic converter for removing HC and CO, the air-fuel mixture supplied to the engine is set to be richer than that corresponding to the stoichiometric ratio. This means that fuel-cost performance is degraded, giving rise to a problem. On the other hand, when the fresh air feeding is stopped in the high load state of the engine such as experienced in up-hill driving of a motor vehicle, the air-fuel ratio of the mixture is controlled to be greater than that the stoichiometric value (i.e., the mixture becomes lean), involving problems that not only generation of NO.sub.x (nitrogen oxides) increases but also knocking or the like unwanted phenomena are likely to take place. Furthermore, the engine may be overheated to injure or destroy the catalytic converter or other components.
As another example of the exhaust gas purification system in which the fresh air is introduced into the exhaust pipe, there is known an apparatus disclosed in, for example, Japanese Unexamined Patent Application Publication No., 132816/1992 (JP-A-H4-132816).
FIG. 31 shows schematically a structure of the apparatus for controlling the fresh air introduced into the exhaust gas purification system for an internal combustion engine disclosed in the above-mentioned publication. As can be seen in the figure, the engine system is comprised of an internal combustion engine 401, a transmission 402 operatively connected to an output shaft (not shown) of the engine 401, an intake pipe 403 for charging air into the engine 401, an exhaust pipe 404 for discharging engine exhaust gas, and an exhaust gas purification system including a catalytic converter unit 405 disposed in the exhaust pipe 404 for purifying the exhaust gas. A throttle valve, 406 is disposed in the intake pipe 403. FIG. 32 shows a structure of the catalytic converter unit 405. Referring to the figure, the catalytic converter unit 405 is comprised of a container 450 in which an upstream catalytic converter 451 including a three-way catalyst and a downstream catalytic converter 452 including an oxidating catalyst, wherein the upstream converter 451 and the downstream catalyst 452 are separated by a split portion 453.
An air feeding system (also referred to as the secondary air feeding system) for introducing fresh air (also referred to as the secondary air) into the exhaust pipe 404 is arranged between the intake pipe 403 and the exhaust pipe 404 and includes an air supply conduit 409 having one end connected to the intake pipe 403 at a position downstream of the throttle valve 406, and the other end connected to a change-over valve 410. In order to prevent the exhaust gas from flowing reversely into the air supply conduit 409, a check valve 412 is mounted in the air supply conduit 409. For controlling the flow rate of the air introduced into the exhaust pipe 404, a control valve 411 is disposed in the air supply conduit 409 at a position upstream of the check valve 412. A first air feeding pipe 413 is connected between the change-over valve (i.e., three-way valve) 410 and the exhaust pipe 404, while a second air feeding pipe 414 is connected between the change-over valve 410 and the split portion 453 of the catalytic converter 405. Furthermore, a negative pressure conduit 415 extends from the intake pipe 403 to the change-over valve 410 via solenoid valves 416 and 417, respectively. The control valve 411, the solenoid valves 416 and 417 are adapted to be controlled by a controller 418 for operation of which electric energy is supplied from a battery 419.
For the operation of the engine 401, a mixture gas of air supplied through the air cleaner 407 and the intake pipe 403 and fuel charged through a fuel injector system (not shown) is charged into the engine 401. At that time, the air-fuel ratio of the mixture gas is regulated by the throttle valve 406, whereby the output torque of the engine 401 is regulated correspondingly. The output of the engine 401 is transmitted to driving wheels (not shown) of a motor vehicle by way of the transmission 402. At a time point immediately after the start of the engine 401, the air-fuel ratio is controlled to be small, as a result of which amounts of carbon monoxide (CO) and hydrocarbon contained in the exhaust gas of the engine 401 is relatively large, while the temperature of the catalytic converter 405 has not attained a level which is sufficiently high for the chemical purification reaction.
Thus, in the state mentioned above, the mechanical air pump 408 is driven by a power derived from the output of the engine 401 via a belt transmission mechanism or the like (not shown), whereby fresh air is supplied to the air supply conduit 409. At this time point, the solenoid valve 416 is opened by the controller 418, as a result of which a negative pressure prevailing within the intake pipe 403 is applied to the change-over valve 410 by way of the negative pressure suction pipe 415 and the solenoid valve 416, whereby the air supply conduit 409 and the first air feeding pipe 413 are placed in fluidal communication with each other. Thus, the fresh air is introduced into the exhaust pipe 404. As a consequence, the exhaust gas becomes rich in oxygen to promote oxidation reaction through the medium of the upstream catalyst 451 and the downstream catalyst 452, which results in that CO and HC are converted into CO.sub.2 (carbon dioxide) and H.sub.2 O (water) to be thereby removed from the exhaust gas before it is discharged into the atmosphere.
Thereafter, when the temperature of the upstream catalyst 451 and the downstream catalyst 452 has reached a level sufficiently high for the exhaust gas purifying reaction, the controller 418 closes the solenoid valve 416 while opening the solenoid valve 417. Then, the negative pressure within the intake pipe 403 is applied to the change-over valve 410 via the negative pressure pipe 415 and the solenoid valve 417. Since the air supply conduit 409 and the second air feeding pipe 414 are placed in fluidal communication with each other at this time point, the fresh air is introduced to the split portion 453 of the catalytic converter 405, as a result of which only the downstream catalyst 452 is exposed to the excess oxygen atmosphere. In this state, nitrogen oxides (NO.sub.x), CO and HC contained in the exhaust gas are first removed through the medium of the three-way catalyst 451 located upstream, which is then followed by removal of remaining CO and HC through oxidizing reaction in the downstream catalyst 452.
In the operation described above, it has been assumed that the fresh air is supplied to only one of the first air feeding pipe 413 and the second air feeding pipe 414 by turning on/off the solenoid valve 416 and the solenoid valve 417. However, in most practical applications, the change-over valve 410 is controlled to an intermediate position so that the fresh air is supplied to both the first air feeding pipe 413 and the second air feeding pipe 414. Thus, the flow rate of the fresh air introduced to the exhaust pipe 404 and the split portion 453 are maintained substantially constant, as is illustrated in FIG. 33.
Owing to the introduction of the fresh air (secondary air) to the exhaust gas purification system, noxious gases such as CO and HC contained in the exhaust gas of the engine 1 is suppressed from being discharged to the atmosphere even in the operation state of the engine 401 which immediately follows the start thereof.
The apparatus for controlling the fresh air feeding to the exhaust gas purification system of the engine described above is however disadvantageous in that since the fresh air of lower temperature than that of the exhaust gas of the engine 401 is introduced into the exhaust pipe 404, the temperature of the exhaust gas as well as that of the upstream catalyst 451 and the downstream catalyst 452 is lowered, involving degradation in the exhaust gas purifying reaction of the catalytic converter unit 405.
It is further noted that the amount of fresh air introduced into the split portion 453 of the catalytic converter 405 may be smaller than that of the fresh air introduced into the exhaust pipe 404. This is because the fresh air introduced into the split portion 453 is demanded only for placing the downstream catalyst 452 in the oxidizing atmosphere. On the other hand, the fresh air charged through the mechanical air pump 408 is at a substantially constant flow rate, as can be seen in FIG. 33. However, the amount of the air charged through the mechanical air pump 408 becomes considerably large widen the air is to be introduced into both the exhaust pipe 404 and the split portion 453 simultaneously, which requires a large capacity for the mechanical air pump 408, to expensiveness. Besides, in order to realize an optimal control of the fresh air introduced to the split portion 453 at a demanded small flow rate, a controller of complicated and expensive structure will be required, thus giving rise to another problem that a complicate and expensive fresh air feed control apparatus of a large scale is required.
In conjunction with a three-way catalyst, it is known that the purification efficiency thereof at a low temperature can considerably be enhanced by varying periodically the O.sub.2 -content of the gas mixture within the catalytic converter. FIG. 34 is a graphical representation of purification efficiency characteristics of a three-way catalyst. In the figure, temperature of the catalyst is taken along the abscissa with CO-purification efficiency along the ordinate. In the figure, a curve A represents a purification characteristic in the case where the fresh air is continuously fed to the three-way catalytic converter. On the other hand, curves B, C, D and E represent corresponding characteristics in the cases where the air is introduced periodically at intervals of 1 sec, 5 sec, 10 sec, 20 sec, respectively. As can be seen, when the air is continuously added to the exhaust gas, purification of CO increases progressively, starting from the catalyst temperature of about 150.degree. C. On the other hand, when the air is added intermittently or in a pulse-like manner, the temperature at which CO can be oxidized becomes low.
It is known from, for example, Japanese Unexamined Patent Application Publication No. 81814/1978 (JP-A-53-81814) that a pressure-responsive valve which is controlled on the basis of an engine intake air pressure and an air-pump discharge pressure is installed in a fresh air feeding pipe to thereby effect intermittently the air introduction to a three-way catalytic converter so that the Gases rich and lean in oxygen content are alternately supplied to the three-way catalytic converter at least five times per second, to thereby promote removal of CO and HC.
However, the known exhaust Gas purification apparatus described just above also suffers problems that the change-over mechanism required for switching the air feeding to the exhaust system is much complicated and demands constituent parts of high precision. Further, difficulty is encountered in generation of control signals as required. Furthermore, because the fresh air of a lower temperature than that of the exhaust gas is added, temperature of the exhaust gas as well as that of the catalyst tends to be lowered, leading unwantedly to degradation in the exhaust gas purifying reaction of the catalytic converter.
As other exhaust gas purification systems in which fresh air is introduced into the exhaust pipe for promoting the purification efficiency of the catalytic converter at an earlier stage of engine operation and in which apparatus is provided for detecting abnormality in the control operations of various components of the system, there may be mentioned those disclosed in Japanese Unexamined Patent Publications Nos. 143362/1988, (JP-A-63-143362), 248908/1988 (JP-A-63 -248908) and 1443/1992 (JP-A-H4-1443).
FIG. 35 shows schematically a general arrangement of an apparatus for introducing heated air into an exhaust system of an internal combustion engine. Referring to the figure, air is drawn at a predetermined flow rate through a motor-driven air pump 608 simultaneously with or after lapse of a predetermined time from the start of the engine 601. The air is then fed into an exhaust pipe 604 of the engine upstream of a catalytic converter 605 from the air pump 608 through an air supply conduit 610, an air flow control valve 613, a check valve 611, a heater 612 and an air feeding pipe 609. The heater 612 is electrically energized under the control of a control unit 614 simultaneously with or after lapse of a predetermined time from the engine start, whereby the air is heated before being introduced into the exhaust pipe 604. Thus, the heated air is fed into the engine exhaust system at a predetermined flow rate at an earlier stage of engine operation.
The heated air fed into the exhaust pipe contributes to activation of catalyst contained in the catalytic converter 605 for thereby promoting the activity of the catalyst to remove noxious gas components HC (hydrocarbon) and CO (oxygen monoxide).
However, in the exhaust gas purification systems equipped with the fresh air (or secondary air) introduction control apparatus, inclusive of those described hereinbefore, there remain problems to be solved that the temperature of the heater may rise steeply to injure or damage not only the heater itself but also peripheral devices disposed in the vicinity thereof when the air flow rate flowing through the heater decreases due to troubles, for example, in the check valve brought about by deposition of sludge contained in the exhaust gas, clogging of the air pump or air cleaner disposed upstream thereof, leakage from the air feeding conduit or pipe, failure of the flow control valve and so forth.